In general, a coating is applied onto the surface of a metal material for protecting the material from corrosion and to maintain its beautiful appearance during its use. Here, in cases where parts such as automobiles and electrical equipments having a pocket portion are coated, electrodeposition is widely applied in primer coating since it has superior deposition property and is low in environmental pollution when compared with air spray coating and electrostatic spray coating. In particular, a cationic electrodeposition coating is widely used as a coating method for primer coating with respect to objects to be coated that are large and require high corrosion resistance such as a body of automobiles. This is since the cationic electrodeposition coating enables sequential coating.
In general, cationic electrodeposition coating is conducted by dispersing a binder component containing a cationic resin and a curing agent in an aqueous medium containing a neutralizer such as an organic acid to give a cationic electrodeposition coating composition, followed by allowing an object to be coated be immersed in the composition as an anode, and then applying voltage.
When the voltage is applied between the electrodes during the coating process, an electrodeposition coating precipitates at the surface of the anode (object to be coated) as a result of an electro-chemical reaction. The electrodeposition coating thus formed contains the curing agent as well as the cationic resin. Accordingly, when the electrodeposition coating is baked after the electrodeposition process, the coating cures and forms a desired cured coating.
As the cationic resin used for the cationic electrodeposition coating composition, amine-modified epoxy resins have been used in the light of corrosion resistance. As the curing agent used for the cationic electrodeposition coating composition, blocked polyisocyanate and the like, a polyisocyanate blocked with alcohol and the like for example, have been used.
In addition, in order to improve the curability, which serves as an indication of various performances of the coating, catalyst that accelerate the cross-linking reaction of the curing agent has been added. Typical catalyst used is an organic tin compound.
However, the organic tin compound may be a cause of catalyst poisoning for the deodorizing catalyst in the baking furnace of the coating line. In addition, from the current environmental regulation trend, there is a possibility that the use of organic tin compounds may be banned. Therefore, development of a cationic electrodeposition coating composition which uses a catalyst alternative to the organic tin compound has been strongly desired.
As the alternative catalyst to the organic tin compound, a cationic electrodeposition coating composition using a zinc borate, a quaternary ammonium salt of organic acid, a zinc compound or the like has been suggested (Patent Documents 1 to 3). However, the effect of these compounds as a catalyst were insufficient, and the curability and the anticorrosion property were not practically satisfactory.
In addition, a cationic electrodeposition coating composition containing a metal chelate compound; a tetravalent organic titanium, zirconium, or hafnium complex having an oxygen-containing ligand; and fluorometalates of zirconium or titanium; have been suggested (Patent Documents 4 to 6). However, curability of the coating was not sufficient when these compounds were used alone, and thus the organic tin compound need be used in combination.
Further, a bismuth acetylacetone complex has been suggested (Patent Document 7), however, since the shelf stability in the paste dispersion is low, the curing property of the catalyst decreases drastically after a certain period of storage, or the viscosity of the composition increases to form a gel, which are regarded problematic.
Accordingly, there has not been known a cationic electrodeposition coating composition which does not contain organic tin compound and can sustain a superior coating curability under currently used baking conditions.